Contacting processes for the purpose of exposing a first material to components contained in a second material are widely used. Some examples of such processes include treating photographic materials with photographic processing solutions; etching metal parts in acid baths; tanning leather; washing, bleaching, and dying of fabrics; and metal plating. These processes can be single stage batch contacting processes, but when practiced on a large scale, are generally single or multi-stage continuous contacting processes. The stages, as defined herein, are either actual discrete areas of contact or mathematically computed theoretical stages in a continuous contacting system which mathematically correspond to actual discrete stages.
When the nature of the materials to be contacted is such that the first material carries some of the second material out of the process, some of the components of the second material are lost. This loss is referred to hereinafter as "carryout". Thus, when the first material is a solid and the second material is a liquid, carryout as referred to herein means a loss that occurs when liquid and its associated components are carried out of the process with the solid.
When the nature of the materials is such that components of the second material are utilized in a reaction with at least some part of the first material, the concept of chemical efficiency is useful in evaluating the performance of a contacting process.
The effects of a difference in chemical efficiency between two continuous steady-state prior art contacting processes can be seen by comparing a known single stage contacting process with a known cocurrent multistage contacting process. While any types of material may be contacted in these processes, for the sake of convenience, the processes will be compared with reference to their use in exposing a solid first material to components contained in a liquid second material.
In the single stage process, the solid and liquid are fed at certain rates into a single well-mixed stage, which has uniform concentrations of liquid components throughout the stage. Since the stage is at steady state, the rates of solid and liquid leaving the stage plus any usage of materials or components thereof due to chemical reactions, evaporation, etc. . . . must equal the rates of materials entering the stage.
In the cocurrent multistage process, a series of stages such as the single stage described above is used. The solid and liquid are fed at certain rates into the first well-mixed stage of the series. The liquid and solid leving the first stage are fed into the second stage. The liquid and solid leaving the second stage are fed into the third stage and so on until the liquid and solid leave the last stage of the process.
For purposes of comparing the chemical efficiency of single stage and cocurrent multistage processes, it will be assumed that there is usage of one of the components in the liquid by reaction with the solid or one of its components and that the rate of reaction is proportional to the concentration of the reacting materials. Furthermore, it will be assumed that the reaction is occurring in each stage of the cocurrent process. In other words, the reaction, if it goes to completion, does not do so until the last stage of the process. If this last assumption were not made, every stage after the stage in which the reaction went to completion in a cocurrent process would be redundant (with no changes occurring from stage to stage) and unnecessary for the analysis.
"Reaction products", as referred to herein, include any components whose concentrations are intended to be increased in one of the materials due to contacting. The term is intended herein to include not only products of chemical reactions, but also components whose concentrations are increased in one material as they desirably transfer to that material from the other material. For example, a "reaction product" with respect to the solid may be a component whose concentration in the liquid is reduced as the component is absorbed by the solid. Thus, with respect to the liquid, that same component may be considered a "reactant", a term intended to include any component whose concentration is desirably decreased in one of the materials due to the contacting process. "Reactants" include components whose concentration in one material is decreased by desired chemical reaction, desired transfer to the other material, or both.
Given the described processes and assumptions, the cocurrent multistage process is more chemically efficient that the single stage process. The definition of "more chemically efficient" will be discussed hereinbelow in an analysis of three variables: (1) contacting time (i.e., the amount of time the solid must spend in contact with the liquid in the process), (2) required input rates of materials and components thereof (i.e., solid input or liquid replenishment) to achieve the desired output of products, and (3) the percent completion of reaction achieved. The analysis is performed by holding two of the variables constant and observing how the third variable changes between one system and the other. Beneficial changes in such variables occur when one changes to a more chemically efficient process.
If the percent completion of reaction and the input rates are held constant, the required contacting time will be shorter in the cocurrent process than in the single stage process. This means that less total material need be actually in the cocurrent process at any instant, as compared to the single stage process. This allows concomitant benefits of less equipment or smaller and/or less complicated equipment and easier startup of the process. A simple mass balance indicates that under these conditions, carryout and the amount of components in the materials leaving the processes are the same for both processes. Alternatively, the shorter contacting time may be realized by increasing both the input rates of materials or components thereof and the output rates of the desired products.
If the contacting time and the input rates are held constant, cocurrent processing will achieve a higher percent completion of reaction than single stage processing. This has the additional effect of causing lower carryout of reactants, higher production and carryout of reaction products, and lower reactant concentration in the materials leaving the process than in the single stage process.
If the percent completion of reaction and the contacting time are held constant, the input rates required to achieve the same reaction product rates will be lower for the cocurrent process than for the single stage process. The lower input rates of the cocurrent process are achieved by lowering the flow rates of the materials or by lowering the concentrations of the reactants in the materials. In each case, reactant carryout and the amount of reactants in the materials leaving the process will be lower in the cocurrent process.
Another continuous multistage prior art contacting process is the counter-current process. In this process, the solid is introduced into the first of a series of stages in which it contacts the liquid. The solid that leaves the first stage enters the second and so on to the last stage of the series from which it leaves the process. The liquid replenishment is introduced into the last stage of the series, and then flows into the next to last stage and so on until it enters the first stage of the series, from which it leaves the process.
The counter-current process has a higher carryout loss of liquid reactants than either the cocurrent or single stage processes. It has a higher chemical efficiency than the single stage process, thus providing the same types of benefits as discussed above for the cocurrent process in the cocurrent vs. single stage analysis. This advantage of the counter-current process over the single stage process in chemical efficiency can, as discussed above, tend to decrease somewhat the higher carryout loss under some operating conditions. The counter-current process does not have a clear advantage or disadvantage in chemical efficiency when compared to the cocurrent process. The results of such a comparison vary with the particular type of reaction occurring, properties of the materials involved and operating parameters chosen.
In all multistage contacting systems, chemical efficiency tends to be increased by increasing the number of stages even though the total contacting time remains the same. This is the primary reason for the relatively high chemical efficiency of cocurrent and counter-current processing as opposed to single stage processing. This gain in efficiency is taken to its theoretical limit when an infinite number of stages are used with an infinitesimally small change in component concentration between stages.
Attempts have been made to decrease chemical carryout losses in the above-described processes. Various well-known methods to decrease the amount of a liquid carried out of the process on a solid have been used in the photographic processing field. These methods include the use of a squeegee coupled with a runoff into the processing tank to reduce the amount of processing solution carried out on the photographic film and the use of an air knife to reduce the amount of liquid clinging to the film. These techniques do not prevent chemical component losses due to liquid being absorbed by the solid and are relatively ineffective if the solid is of a shape that does not lend itself well to a physical scraping or cleaning.
Techniques involving rinsing a solid after contacting coupling with a recovery of the components rinsed off the solid have also been used to decrease chemical component losses. U.S. Pat. No. 3,329,542 discloses as prior art the use, in etching of metal in acid baths, of a pre-rinse tank after the metal is etched in the acid bath, but before it undergoes final rinsing. The contents of the pre-rinse tank are then used for replenishment of the etching bath. The process disclosed as the invention of the above-mentioned patent involves spraying metal wire or ribbon with water as it emerges from the etching bath. The wire or ribbon is given helical turns so that the rinse spray runs down the wire or ribbon into the etching bath. The amount of water sprayed onto the metal corresponds to the amount of water loss from the etching tank. Rinsing techniques may, however, be ineffective against carryout loss due to absorption of the liquid into the solid. Additionally the above-described techniques all require the addition to the process of extra equipment and are often difficult to operate and control.
Clearly, there is a need for a contacting process that provides high chemical efficiency and low carryout of valuable components in the liquid carried out of the process with the solid. Such a system could be used by itself or in conjunction with the above-described techniques for reducing carryout loss. It is toward this objective that the present invention is directed.